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The present invention relates to customer premises telecommunications systems and more particularly to a system which uses existing customer premises wiring for both POTS and DSL services without interference with local exchange carrier POTS service.
The common telephone service provided to customer premises, including homes and businesses, is analog service provided over copper wires usually referred to as twisted pair. This service is referred to as POTS, or plain old telephone system, service. It is sometimes considered to be synonymous with the public switched telephone network, PSTN. Most of the PSTN now carries signals digitally. However, POTS service from the local exchange carrier, LEC, central office, CO, to customer premises is analog and carried over copper wire. In addition to voice, the tip and ring signals and power to operate telephone sets in the customer premises are carried over the copper wires. An advantage of this system is that the telephone system does not depend on having power available at the customer premises. The CO normally uses standard grid power to operate and has emergency power backup systems which keep the telephone system operating in emergency situations.
Digital subscriber loops or lines, DSL, were developed to provide digital data service over the same twisted pair lines which are used for POTS. This type of service has great advantage when customers connect computers to the Internet or other networks from their homes or small business premises. Both POTS service and DSL service can be provided over the same copper wires so that no additional expense of running extra lines for DSL is required. POTS service operates at frequencies below 4 KHz while DSL operates at frequencies above 4 KHz up to several MHz. It is therefore a simple matter to separate the signals by frequency filters.
As the DSL service has become more available, it is becoming common to provide voice over DSL, VoDSL, service. In this type of service, the analog telephones in a customer premises are connected to a device, e.g. a VoDSL Integrated Access Device (IAD), which converts the analog telephone signals to digital signals and sends them to the CO over a DSL connection. An example of an IAD is the CPE, customer premises equipment, described in U.S. Pat. No. 6,272,209 issued on Aug. 7, 2001. This arrangement has a number of advantages. For example, a number of separate telephone voice signals can be transmitted simultaneously over one DSL connection. Data signals may also be transmitted over the DSL connection at the same time. There is no need to run additional twisted pairs when a customer wants additional telephones or additional computer connections. This arrangement effectively extends the PSTN digital system past the CO and into the customer premises.
VoDSL systems must have a source of electrical power. It has not proven practical to provide power from the CO as is done for POTS service. The power requirements are too great. As a result, the systems use power from the customer premises. For emergency purposes, the systems may have batteries for backup when the AC power is lost. However, such systems must be maintained by the customer to be sure that the backup batteries are charged and are replaced at regular intervals. In any case, backup batteries have a limited amount of power available and will run down after a long outage of AC power.
As VoDSL systems become more common, more customers want to perform their own installation. This requires that the system be easy and simple to install. Such systems should be designed to plug into existing wiring to the extent possible. Running new wiring in a customer premises requires a major effort which most customers would not want to undertake. It is also desirable that an IAD be located close to a customer""s personal computer, since they often have a network connection directly to the computer. However, in existing systems IADs must be connected between the copper wires entering the premises and the internal wires connected to the telephone sets. This normally requires breaking or interrupting the wiring at or near a location on the exterior of the customer premises. The analog signals between the IAD and the telephone sets in the customer premises must be isolated from the POTS service connection to the CO.
It would be desirable to have a voice over DSL system which is simple to install, does not require additional wiring in the customer premises and which provides telephone service when the VoDSL system loses power.
A voice over DSL system according to the present invention includes an improved network interface device, conventional customer premises wiring, and an integrated access device having both DSL and POTS ports coupled to the customer premises wiring. The improved network interface device includes a filter which couples DSL frequencies across the network interface device at all times and an automatic switch which opens when an integrated access device is operating to isolate POTS service in the customer premises from POTS service in a local exchange carrier central office.
In one embodiment, the invention includes a DSL blocking filter between a telephone set and its connection to the customer premises telephone wiring.